Science & Technology

A cosmic explosion in 2022 was the brightest ever — and may overturn decades-long theory on gamma ray bursts

New analysis by Pune institute shares more possibilities of reasons behind fast-moving jets that produce X-ray and visible light in the afterglow

By Himanshu Nitnaware
Published: Wednesday 29 March 2023
The Giant Metrewave Radio Telescope at National Centre for Radio Astrophysics (NCRA), Pune was among the instruments that recorded the bright gamma-ray radiation in October 2022. Photo: @NCRA_Outreach / Twitter

The brightest gamma-ray radiation of all time was recorded by astronomers around the world in October 2022. Now, data from the radiation pulse may help overturn a decades-long theory of gamma ray burst (GRB) jets, according to a new study. 

The 5-minute-long radiation recorded on October 9, 2022 was the most intense ever, causing disruptions in radio signals on earth and cutting across our solar system. Astronomers, including ones from the home of radio astronomy in India, National Centre for Radio Astrophysics (NCRA), Pune, concluded such signals were a birth cry of a black hole. 

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The new study appeared in Astrophysical Journal Letters, and observations of the radiation named GRB 221009A throw a fresh light on the long quest to fathom the origin of the noted extreme cosmic explosions. 

The observation was conducted jointly by Center for Astrophysics, Harvard and Smithsonian’s Submillimeter Array (SMA) in Hawaii, the MeerKAT Array in South Africa, the US National Science Foundation’s Karl G Jansky Very Large Array (VLA) in New Mexico (USA), the Atacama Large Millimeter Array (ALMA) in Chile and NCRA’s Giant Metrewave Radio Telescope, India.

“Long duration” GRBs were the outcome of a birth of a black hole formed as the core of a massive star falling under its weight, according to the scientists. The phenomenon of the formation of a new black hole releases powerful plasma jets almost of the speed of light shooting gamma rays.

“As the jets slam into the gas neighbouring the dying star, a bright afterglow is produced across the spectrum,” said Tanmoy Laskar, assistant professor of Physics and Astronomy at the University of Utah and lead author of the findings said in a press statement released by NCRA. 

Analysing the data from all the telescopes, the astronomers claimed the radio signals from the afterglow were brighter than expected.

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“At first, we were very excited because we thought we might have captured the fleeting signature of a ‘reverse shock’ — a shock wave going backwards through the jet and lighting it up in the radio,” said Kate Alexander, assistant professor of Astronomy at the University of Arizona and co-author of the study.

Such signatures rarely get detected and finding the brightest GRB could have helped the researchers pinpoint the composition of the GRV’s jets, a riddle that is weakly understood by the community, the astronomers said. 

“What we found in the radio looks a bit like a reverse shock in the sense that there is extra emission at radio frequencies,” Laskar said.

“But we also have models for how a reverse shock spectrum should evolve with time, and the radio spectrum of GRB 221009A faded much too slowly. So either we don’t understand reverse shocks, or we’ve found a completely new emission component,” the author added.

The data observed by studying the emission component’s peak frequency and peak brightness points helped to calculate the outflow, which may consist of a small mass shooting at 99.4 per cent at the speed of light, said Alexander. 

Though the mass is fast for most astrophysical objects, it is comparatively slower than GRB jets, said Raffaella Margutti, associate professor of astronomy and physics at the University of California, Berkeley and co-author of the study.

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“There is a possibility of a very fast-moving jet that is producing the X-ray and visible light in the afterglow. But our modelling suggests that something else entirely is creating the radio light,” Margutti said, adding that it may lead to overturning the astronomers’ understanding of how GRB jets produce light. 

Irrespective of the outcome, the findings challenge the decades-old theory on GRB jets and need reassessment, Margutti added.

“We still don’t know what is producing the radio emission, but we are still collecting data and it’s possible that some more theoretical investigations and numerical simulations together could help us in deriving a better understanding. But for now, it’s still a bit of a mystery,” she said.

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